Conventionally, in, for example, automobile electric components such as automobile ECUs (Electrical Control Units), motors, lamps, and sensors, household electric appliances such as electric toothbrushes, shavers, and cellular phones, and solar cells, an opening is provided in a housing that accommodates an electronic component, a control board, etc., in order to eliminate a pressure difference between the inside and outside of the housing, and the opening is covered with a water-proof air-permeable filter. This water-proof air-permeable filter ensures ventilation between the inside and outside of the housing and also prevents foreign matters such as water and dust from entering the housing.
For such a water-proof air-permeable filter, a polytetrafluoroethylene (PTFE) porous membrane having favorable air permeability and high water pressure resistance is commonly used (for example, see Patent Literature 1). Generally, a PTFE porous membrane can be obtained by molding PTFE fine powder into a sheet shape and then stretching the sheet-shaped molded body in two directions orthogonal to each other.